Microstructure evolution and nonequilibrium phase diagram for Ni-Hf binary alloy produced by laser cladding

Synthesis of nonequilibrium Ni-Hf binary alloys were carried out using laser cladding technique. In this process mixed powder in the ratio of Ni-26 wt% Hf was delivered using a screw feeder into a melt pool of the substrate, generated by a high power continuous wave CO₂ laser beam. The microstructure of the claddings thus produced were investigated using optical, scanning and transmission electron microscopy and X-ray microanalysis techniques. Due to the inherent rapid cooling rate associated with the process of laser cladding process, some nonequilibrium hereto unreported phases formed in the claddings. There is also an extension in the solid solubility of Hf in the terminal α phase as compared to the equilibrium Ni-Hf binary phase diagram. This paper investigates this solid solubility extension, the evolution of the microstructure in the claddings in the system and also characterizes the metastable phases formed in terms of crystal structure and microchemistry. A nonequilibrium phase diagram for Ni-Hf binary alloy is recommended based on the micro-chemistry and differential thermal analysis data.     

Effect of Mg on Solidification Microstructures of Homogenous and Functionally Graded A390 Aluminum Alloys

Hypereutectic Al?CSi alloys are used in components that require high resistance wear and corrosion, good mechanical properties, low thermal expansion and less density. The size and morphology of hard primary silicon particles present in Al?CSi alloys greatly influences the mechanical properties. Addition of Mg leads to formation of intermetallic Mg₂Si phases, which contributes towards the properties of high silicon alloy as well as alters the nature and quantity of primary silicon formed. The high silicon alloy subjected to centrifugal casting leads to the formation of functionally gradient material, which provides variation in spatial and continuous distribution of primary phases in a definite direction exhibiting selective properties and functions within a component. The present study is to evaluate the effect of Mg on solidification microstructures of homogenous and functionally graded A390 aluminium alloys. The addition of Mg from 3 to 5?% in A390 alloy using Al?C20Mg master alloy has shown a transformation from primary silicon rich matrix to Mg₂Si rich matrix. Centrifugal casting shows the gradient distribution of primary silicon and Mg₂Si phases towards the inner periphery of the casting.     

Modeling of Heat Transfer and Fluid Flow in the Laser Multilayered Cladding Process

The current work examines the heat-and-mass transfer process in the laser multilayered cladding of H13 tool steel powder by numerical modeling and experimental validation. A multiphase transient model is developed to investigate the evolution of the temperature field and flow velocity of the liquid phase in the molten pool. The solid region of the substrate and solidified clad, the liquid region of the melted clad material, and the gas region of the surrounding air are included. In this model, a level-set method is used to track the free surface motion of the molten pool with the powder material feeding and scanning of the laser beam. An enthalpy–porosity approach is applied to deal with the solidification and melting that occurs in the cladding process. Moreover, the laser heat input and heat losses from the forced convection and heat radiation that occurs on the top surface of the deposited layer are incorporated into the source term of the governing equations. The effects of the laser power, scanning speed, and powder-feed rate on the dilution and height of the multilayered clad are investigated based on the numerical model and experimental measurements. The results show that an increase of the laser power and powder feed rate, or a reduction of the scanning speed, can increase the clad height and directly influence the remelted depth of each layer of deposition. The numerical results have a qualitative agreement with the experimental measurements.     

Interaction coefficients in Fe-C-Ti-i (i = Si,Cr, AI,Ni) systems

Ever since Schroeder and Chipman_[10] initiated the application of the silver bath iso-activity method to the study of activity interaction coefficients of components in iron alloys, the method was only applied to some ternary systems in which only one component dissolves in both liquid iron and liquid silver. The authors of the present work proposed a method which enables the silver bath iso-activity method to be utilized to study the activity interaction coefficients of two components simultaneously dissolvable in both liquid iron and liquid silver by establishing an iso-i-j-activity state for several Fe-C-i-j quarternary samples through a common silver bath. This new “quarternary silver bath iso-activity method” was applied to quarternary Fe-C-Ti-i (i = Si,Cr, Al,Ni) melts at 1600 °C and estimated the activity interaction coefficients as follows: ε _Ti ^¡ = 7.96, ρ _Ti ^¡ = ?6.88, ρ _si ^Ti = 0.51, ρ _Ti ^Ti,si = ?2.27, ρ _Ti ^Ti,si = ?5.66 ε _Ti ^? = 3.46, ρ _Ti ^? = 10.43, ρ _Ti ^Cr,Ti = 17.40 ε _Ti ^Al = 0.93, ρ _Ti ^? = 10.22, ρ _Ti ^Al,Ti = 25.11 ε _Ti ^Ni = 2.49, ρ _Ti ^Ni = 9.33, ρ _Ti ^Ni,Ti = 16.37     

镁合金表面激光电弧复合熔覆 5556 铝合金工艺研究

铝合金熔覆是轻量化镁合金表面涂层防护的重要方法。 本研究使用 AZ80A 镁合金作为基材, 使用 5556 铝 合金作为熔覆合金, 并使用激光电弧复合熔覆进行了铝合金熔覆层制备。 对熔覆层组织进行了分析, 重点研究了 激光摆动对熔覆层品质的影响。 结果表明, 当激光无摆动时, 熔覆层宽度有限, 无法正常形成各道次熔覆层的有 效搭接, 且缺陷较多。 在增加激光摆幅的情况下, 激光加热能量会更均匀地在镁合金基材表面分散, 有效增加了 熔宽, 提升了各道次熔覆层的搭接率, 促进内部缺陷更少、 品质更高的连续熔覆层的形成。     

增材制造TiAl基合金的研究进展

TiAl基合金具有优异的高温性能,是一种极具竞争力的新型轻质高温结构材料,在汽车、军工、航空航天等领域具有广阔的发展潜力和应用前景.然而,TiAl基合金室温脆性较大,成形困难,是阻碍其发展与应用的主要瓶颈之一.增材制造基于"离散+堆积"的成形思想,以激光、电子束、电弧等作为高能热源,通过熔化丝材或者粉末,逐层堆积实现零...     

Microstructure and Wear Resistance of Laser Clad Cobalt-Based Alloy／SiCp Composite Coating

The SiCp (20%) reinforced cobalt-based alloy composite coatings deposited by lasercladding on IF steel were introduced. The microstructure across the whole section of such coat-ings was examined using optical microscope, scanning electron microscope (SEM) and X-raydiffractometer (XRD), and the wear resistance of the coatings was measured by MM-200 typewear testing machine. The results show that the SiCp is completely dissolved during laser clad-ding and the primary phase in the coatings is r-Co. The other phases, such as SizW, CoWSi,Cr3Si and CoSi2, are formed by carbon, silicon reacting with other elements existing in themelting pool. There are various crystallization morphologies in different zones, such as planarcrystallization at the interface, followed by cellular and dendrite crystallization from interface tothe surface. The direction of solidification changes from one direction perpendicular to interfaceto multi-directions at the central and upper regions of the clad. The wear resistance of the cladis improved by adding SiCp.     

Microstructure and wear properties

The laser surface cladding technique was used to formin situ Fe-Cr-Mn-C alloys on AISI 1016 steel substrate. In this process mixed powders containing Cr, Mn, and C with a ratio of 10:1:1 were delivered using a screw feed, gravity flow carrier gas aided system into the melt pool generated by a 10 kw CO₂ laser. This technique produced ultrafine microstructure in the clad alloy. The microstructure of the laser surface clad region was investigated by optical, scanning, and transmission electron microscopy and X-ray microanalysis techniques. Microstructural study showed a high degree of grain refinement and an increase in solid solubility of alloying elements which, in turn, produced a fine distribution of complex types of carbide precipitates in the ferrite matrix because of the high cooling rate. An alloy of this composition does not show any martensitic or retained austenite phase. In preliminary wear studies the laser clad Fe-Cr-Mn-C alloys exhibited far superior wear properties compared to Stellite 6 during block-on-cylinder tests. The improved wear resistance is attributed to the fine distribution of metastable M₆C carbides.     

Effect of Fluid Convection on Dendrite Arm Spacing in Laser Deposition

Ni superalloys are widely used for hot section components in jet engines because they are very resistant to corrosion and maintain reasonably high strength at elevated temperature. However, the repair cost of the parts is high, partly due to the complexities of process variable optimization and control in laser cladding. In particular, optimizing the process parameters by experiments is time-consuming and costly. The microstructure and properties of the metal deposit are significantly influenced by values temperature gradient G and solidification rate R at the weld pool solidification boundary. Optimized values can help to reduce defects and improve properties of laser deposits. Optimization is hindered by the fact that the clad melt pool is hot and small, making in situ measurement of such solidification conditions difficult. Numerical simulation of the laser deposition process is a possible alternative to experimental measurement to obtain values of clad solidification parameters. In this investigation, G and R values at the weld pool solidification boundary were obtained from a three dimensional numerical simulation of laser deposition process and melt pool. The primary dendrite arm spacing and cooling rate of the deposited material were then correlated to these solidification conditions.     

Microstructure and wear properties of laser clad Fe−Cr−Mn−C alloys

The laser surface cladding technique was used to formin situ Fe-Cr-Mn-C alloys on AISI 1016 steel substrate. In this process mixed powders containing Cr, Mn, and C with a ratio of 10∶1∶1 were delivered using a screw feed, gravity flow carrier gas aided system into the melt pool generated by a 10 kw CO₂ laser. This technique produced ultrafine microstructure in the clad alloy. The microstructure of the laser surface clad region was investigated by optical, scanning, and transmission electron microscopy and X-ray microanalysis techniques. Microstructural study showed a high degree of grain refinement and an increase in solid solubility of alloying elements which, in turn, produced a fine distribution of complex types of carbide precipitates in the ferrite matrix because of the high cooling rate. An alloy of this composition does not show any martensitic or retained austenite phase. In preliminary wear studies the laser clad Fe-Cr-Mn-C alloys exhibited far superior wear properties compared to Stellite 6 during block-on-cylinder tests. The improved wear resistance is attributed to the fine distribution of metastable M₆C carbides.     

Effect of extended solid solution of Hf on the microstructure of the laser clad Ni-Fe-Cr-Al-Hf alloys

Alloys and coatings for alloys for improved high temperature service life under aggressive atmosphere are of great contemporary interest. There is a general consensus that addition of reactive elements such as Hf will provide many beneficial effects for such alloys. The laser cladding technique was used to produce Ni-Fe-Cr-Al-Hf alloys with extended solid solution of Hf. A 10 kW CO₂ laser with mixed powder feed was used for laser cladding. Optical, scanning electron (SEM) and scanning transmission electron (STEM) microscopy were employed for microstructural evolution of alloys produced during laser cladding processes. The electron probe microanalysis and the auger electron spectroscopy were also used for micro-chemical analysis of different phases. Microstructural studies revealed a high degree of grain refinement, considerable increase in solubility of Hf in matrix and Hf rich precipitates and new metastable phases. This paper will report the microstructural development in this laser clad Ni-Fe-Cr-Al-Hf alloy.     

Modeling of laser keyhole welding: Part II. simulation of keyhole evolution,velocity, temperature profile,and experimental verification

This article presents the simulation results of a three-dimensional mathematical model using the level set method for laser-keyhole welding. The details of the model are presented in Part I.^[4] The effects of keyhole formation on the liquid melt pool and, in turn, on the weld bead are investigated in detail. The influence of process parameters, such as laser power and scanning speed is analyzed. This simulation shows very interesting features in the weld pool, such as intrinsic instability of keyholes, role of recoil pressure, and effect of beam scanning. For verification purposes, visualization experiments have been performed to measure melt-pool geometry and surface velocity. The theoretical predictions show a reasonable agreement with the experimental observations.     

A Melt Pool Temperature Model in Laser Powder Bed Fabricated CM247LC Ni Superalloy to Rationalize Crack Formation and Microstructural Inhomogeneities

This study of the laser powder bed fusion (LPBF) of γ′-strengthened Ni superalloy CM247LC focuses on the development of a melt pool temperature model to predict crack density within the alloy. This study also analyzes spatter and elemental evaporation, which might cause defects and inhomogeneities, at different melt pool temperatures. The melt pool temperature model provides more accurate predictions than the widely used energy density model. Spatter particles were collected and characterized to study their sizes and chemical compositions, compared with the virgin powder, recycled powder, and as-built samples, to probe the impact of their entrapment into the melt pool. This study also investigated Al evaporation, revealing that its extent does not correlate with the laser energy density and is believed to be rather limited by comparing the chemistry of the virgin powder and the build. Last, the impact of LPBF process parameters on the formation of these inhomogeneities, and accordingly crack formation, was studied using finite element analysis by estimating the maximum melt pool temperature and correlating it with the formation of the microstructural inhomogeneities. The morphology of the various cracking modes was associated with the process parameters.

     

Effect of chilling and cerium addition on microstructure and cooling curve parameters of Al–14%Si alloy

Abstract

Al–14%Si alloys, with and without cerium, were cast at varying cooling rates by solidifying them in a crucible and against chills. The effect of melt treatment and chilling on microstructure and cooling curve parameters of the alloy was assessed. Ce treated alloys solidified in clay graphite crucible at a slow cooling rate showed refinement of primary silicon and the formation of Al–Si–Ce ternary intermetallic compound. The addition of Ce to the alloy solidified against chills resulted in simultaneous refinement and modification of primary and eutectic silicon. Nucleation temperatures of both primary and eutectic silicon decreased on addition of cerium. The formation of the intermetallic compound decreased with increase in cooling rate, leading to the modification of the eutectic silicon. The increase in the degree of modification of the eutectic Si was associated with the decrease in the volume fraction of the intermetallic compound formed.     

Microstructure of laser clad Ni- Cr- Al- Hf alloy on a γ′ strengthened ni- base superalloy

Alloys and coatings for alloys for improved high temperature service life under aggressive atmo-spheres are of great contemporary interest. There is a general consensus that the addition of rare earths such as Hf will provide many beneficial effects for such alloys. The laser cladding technique was used to produce Ni-Cr-AI-Hf alloys with extended solid solution of Hf. A 10 kW CO₂ laser with mixed powder feed was used for laser cladding. Optical, scanning electron (SEM) and scanning transmission electron (STEM) microscopy were employed to characterize the microstructure of alloys produced during laser cladding processes. Microstructural studies revealed grain refinement, considerable in-crease in solubility of Hf in the matrix, Hf-rich precipitates, and new metastable phases. The size and morphology of γ′ (Ni₃Al) phase were discussed in relation to its microchemistry and the laser processing conditions. This paper will report the microstructural development in this laser clad Ni-Cr-AI-Hf alloy. Formerly Visiting Assistant Professor, Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign.     

Modeling of laser cladding with powder injection

Laser cladding is one of the material additive manufacturing processes used to produce a metallurgically bonded deposition layer. To obtain a high-quality resulting part, a deep understanding of the underlying mechanisms is required. In this article, a mathematical model is developed to simulate the coaxial laser-cladding process with powder injection, which includes laser- substrate, laser-powder, and powder-substrate interactions. The model considers most of the associated phenomena, such as melting, solidification, evaporation, evolution of the free surface, and powder injection. The fluid flow in the melt pool, which is mainly driven by Marangoni shear stress as well as particle impinging, together with the energy balances at the liquid-vapor and the solid-liquid interfaces, are investigated. Powder heating and laser power attenuation due to the powder cloud are incorporated into the model in the calculation of the temperature distribution. The influences of the powder injection on the melt pool shape, penetration, and flow pattern are predicted through the comparison for the cases with powder injection and without powder injection. Dynamic behavior of the melt pool and the formation of the clad are simulated. The effects of the process parameters on the melt pool dimension and peak temperature are further investigated based on the validated model.     

激光熔覆温度场模拟与表征的研究现状

激光熔覆熔池内存在传质、传热、对流及气-液-固界面间的反应,熔覆过程中温度场的变化对熔覆层组织和性能产生重要影响,正确解析这一复杂物理冶金过程,对激光技术发展有重要指导作用。本文回顾了近年国内外激光熔覆温度场的模拟研究进展,重点评述了激光加工温度场从数学分析到数值模拟的研究历程,并针对这一领域今后的发展提出了一些看法。     

Processing,microstructure, and properties of laser-clad Ni alloy FP-5 on Al alloy AA333

This article describes the results of laser cladding Ni alloy FP-5 on Al alloy AA333, microstructure and crystal structure characterization, and properties of the clad evaluated by Vickers hardness measurement and wear testing. Direct cladding of Ni alloy on Al alloy creates brittle Ni _x Al _y compounds in the interface, which make the interface very brittle, and result in cracking at the interface. The compound formation is avoided by introducing an intermediate layer of Cu or bronze. The cracking tendency of the clad is prevented by preheating the substrate to 673 K. The microstructure and crystal structure of the clad and interface are investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Five phases in the clad layer (including three new phases) and two phases in the interface are identified by convergent beam electron diffraction (CBED) and selected area diffraction (SAD) studies. The mechanical properties of the laser-clad Ni alloy are evaluated by Vickers hardness measurements and wear testing, which show superior results over Cu- and Fe-based alloys.     

Simultaneous Decarburization and Oxidation Reactions Occurring in Silicon and Ferrosilicon Alloys at 1823 K

We present high temperature investigations on ferrosilicon alloys (Si: 24.7, 74, and 98.5%); silicon oxidation and decarburization studies in steelmaking have generally been limited to Fe‐based alloys containing up to 3.5 wt% Si. The effect of alloy composition, oxygen partial pressure and gas flow rate on interactions at 1823 K is evaluated. Decarburization and silicon oxidation reactions were found to occur simultaneously with significant differences observed in the weight gain and carbon loss. The net weight gain in these alloys was found to be due to the combined influence of decarburization (weight loss due to the generation of a gaseous product) and silicon oxidation (weight gain due to silica formation on the sample surface). There was a clear evidence for two rate regimes: the rate of decarburization was found to be much higher during the initial 2 min and a much slower rate was observed in later stages for all specimens. These rate regimes are explained in terms of the extent of surface coverage with the reaction product silica. No significant effect was found on the decarburization rates when the proportion of oxidizing gas (CO₂) was increased from 20 to 100%. The outcomes of this investigation will assist with the development of mechanisms governing the reactions of molten ferrosilicon and silicon alloys during their interactions with gaseous phases in the cupola process.